Portable terminal devices, such as a mobile phone and a PHS, including a liquid crystal display are now widely used. Such a portable terminal device generally includes an illuminating device so as to allow a user to observe the liquid crystal display even under a dark environment. As the illuminating device used in the portable terminal device, there is a known one of an optical-guide-plate type that receives light emitted from an LED inside the optical guide plate and emits the received light from the front surface side of the optical guide plate. Along with a reduction in the size and thickness of the portable terminal device, a technique of fixing the optical guide plate onto a frame in a reduced space is now required for the illuminating device incorporated in the portable terminal device.
As a method for fixing the optical guide plate onto the frame in the illuminating device of an optical guide plate type, there is a known one that forms a protrusion member (engaging protrusion) on the side surface of the optical guide plate and fits the engaging protrusion to a recessed portion formed in the frame. FIG. 6(a) is a top plan view of a liquid crystal display device including the illuminating device of this type, and FIG. 6(b) is a sectional view thereof. A frame 201 plays a role for fixing components of the liquid crystal display device. A liquid crystal display panel 202 includes a pair of glass substrates and a liquid crystal layer sandwiched therebetween. In the liquid crystal display panel 202, an electric field is applied to the liquid crystal layer so as to control the orientation of liquid crystal molecules and thereby control the amount of light transmitted therethrough, whereby an image is displayed.
The optical guide plate 204 receives therein light emitted from an LED 207 configuring a light source through a light-incident end surface. The received light is repeatedly reflected and transmitted at a luminescence section of the optical guide plate 204 to be converted into a surface-emission light. The optical guide plate 204 then emits the surface-emission light toward the liquid crystal display panel 202. Engaging protrusions 205 are formed on the side surfaces of the optical guide plate 204 that cross the light-incident end surface at right angles. The engaging protrusions 205 are each formed as a part of the optical guide plate 204 and each have a shape corresponding to the recessed portions formed in the frame 201. The engaging protrusions 205 are fitted in the recessed portions of the frame so as to fix the optical guide plate 204 onto the frame 201.
An optical sheet 203 collects the light emitted from the optical guide plate 204 on the liquid crystal display panel 202. A reflecting sheet 206 reflects the light leaking from the optical guide plate 204 toward the reflecting sheet 206 so as to return the leakage light toward the optical guide plate 204. The light emitted from each LED 207 enters the optical guide plate 204 while spreading in a fan-shape as a light beam 209 shown in FIG. 6(a). An effective display area 208 defines the effective visible range of the display screen of the liquid crystal panel 202.
In the liquid crystal display device 200 shown in FIGS. 6(a) and 6(b), a bright line 210 occurs due to reflection of light at the corner portion of the base of the engaging protrusion 205 formed on the side surface of the optical guide plate 204. The bright line 210 enters the effective display area 208 to degrade the display quality of the liquid crystal display device 200. As a technique to solve the problem of the occurrence of the bright line due to existence of the engaging protrusion, there is a known technique described in WO2004/055430. FIG. 7 shows a liquid crystal display device including the illuminating device described in WO2004/055430. The liquid crystal display device 200a shown in FIG. 7 differs from the liquid crystal display device 200 of FIG. 6 in that the engaging protrusions 205 are formed over the entire area of the side surfaces of the optical guide plate 204. The technique of WO2004/055430 adopts the above configuration to prevent occurrence of the bright line at the corner portion of the base of the engaging protrusions 205.
The problems encountered in the conventional liquid crystal display device are considered in the present invention as follows. In the configuration of the liquid crystal display device of WO2004/055430, since the engaging protrusions 205 are formed over the entire area of the side surfaces of the optical guide plate 204, the longitudinal dimension of the engaging protrusion 205 is increased. Thus, the thickness of the frame 201 needs to be reduced over the entire longitudinal direction thereof at the fitting portion. Therefore, the mechanical strength of the frame 201, which is required to have an enough strength to support components of the liquid crystal display device, is decreased.
Patent Publication JP-1997-243828A also describes an illuminating device that includes positioning protrusions on the side surfaces of the optical guide plate. In the device, protrusions are formed on the light-incident side surfaces of the optical guide plate, and the shape of the protrusions is appropriately selected, whereby an uneven luminance caused due to existence of the protrusion is prevented. However, this publication assumes the use of a fluorescent lamp as a light source for irradiating the light-incident surface with light and does not assume the use of a light source emitting light that spreads in a fan-shape. Therefore, the spread of the light entering the optical guide plate through the light-incident surface, which is caused by the use of a light source emitting light that spreads in a fan-shape, is not considered. Thus, even by adopting the configuration described in JP-1997-243828A, the problem that bright line occurs at the protrusion when the protrusion exists within the outgoing-angle range of light spreading in a fan-shape cannot be solved.